Machine vision is used in an increasing number of manufacturing operations. One role for machine vision (or a “vision system”) is the verification of product quality through inspection. In a typical vision system inspection arrangement, one or more cameras are positioned over a moving line, with appropriate illumination, and images of the line, within the field of view of each camera are acquired and processed. The processing can include a comparison of the acquired image to a trained model using appropriate vision system applications (e.g. edge detection, blob analysis, rotation and scale-invariant search, etc.) that reside on a processor within the camera, or in an interconnected processor. The results of the processing can indicate when a defect is present. These results can be used by a controller to direct various operations, such as issuing alarms and alerts, stopping the moving line and actuating a part-reject mechanism.
FIG. 1 shows a typical vision system inspection arrangement 100 used to inspect for defects on a moving web 110 or similar continuous moving surface. As the web 110 moves (arrow 112) downstream, it passes through at least one inspection station 120 containing at least two cameras Camera 1 (122) and Camera 2 (124). These cameras each define a respective field of view (lines 126, 127 and lines 128, 129) that together covers the entire width WW of the web 110. This can be termed a “cross-web” inspection system. In this example, each field of view overlaps (crossing lines 127, 128) to ensure full acquisition of the web line. The outer edges of the field of view (outer lines 126, 129) extend at least to the opposing edges of the web 110, or to edges of the desired inspection region. The cross-web field of view in this example is illuminated by a bar illuminator 130 that resides on a back side of the web (opposite the camera-facing web side) so as to generate an illumination line (dashed line 132) that projects translucently through the web. The illuminator can be an LED-based illuminator, and can be controlled by an appropriate controller 131. The cameras 122, 124 acquire a stream of images that include the region within the illumination line. Any defects occurring within the line are identified by the vision system application that (in this example) resides in each camera 122, 124. The inspection results IR1 and IR2 of each respective camera 122 and 124 for each image frame are transmitted to a controller and/or processor 140 that directs a desired line operation 150—for example issuing an alert and/or stopping the moving line. The results can also be placed in an inspection file (i.e. a computer-readable file) 142 that is appended to information regarding the web for quality control and other purposes.
The exemplary web inspection arrangement 100, and other surface inspection systems, may operate continuously for days without stoppage. One particular concern is that the system is functioning properly, if at all, after an extended period of runtime. That is, one or both cameras may be malfunctioning and allowing defects to pass undetected. A popular technique for determining continued operation of the inspection cameras is to insert an object, such as an operator's hand, briefly into the field of view of each camera. If the controller processor responds, detecting an obstruction, then the camera is at least nominally functioning.
A further concern is whether the camera system is still within acceptable calibration. Calibration of each camera occurs at setup time, and typically involves skilled technicians. Over time calibration can drift due to internal and external changes in the camera setup. For example vibration or a jar to a camera can cause it to misalign, and lose calibration. If the calibration is sufficiently drifted, then it is desirable to perform a recalibration of the camera system. Likewise, illumination intensity or focus may have diminished or changed in an unacceptable way over time. Due to the need to take the line out of service and the use of skilled technicians, recalibration is best performed when actually needed. Conversely, when recalibration is needed, it should be performed relatively soon so that inspection errors do not occur.
More generally, under various industry quality standards, such as ISO 9001, product quality and equipment performance must be monitored. The ability to regularly and continuously validate the alignment, focus and overall performance of cameras in an inspection system, without affecting the inspection results is a highly desirable and heretofore unachieved goal.
Accordingly, it is desirable to provide a system and method for allowing the state of vision system function to be determined in a surface inspection arrangement. The system and method should desirably allow the present calibration to be determined. Such determination should be performed during runtime without stopping the line or generally degrading ongoing inspection activities.